The transmission capacity of optical wavelength division multiplexed channels in a WDM transmission system can be increased by reducing the frequency separation between the individual optical channels. The spectral efficiency of the transmission system increases with a continual reduction in the frequency separation and simultaneous increase in the channel data rate. At the same time, the demands imposed on the optical filters which separate the individual optical channels from one another also increase. On the one hand, such optical filters must allow the wanted signal to pass unimpeded as far as possible but, on the other hand, interference from the adjacent channels should be suppressed as efficiently as possible.
An optical channel selected in this manner arrives at a receiver which converts the amplitude-modulated light to a sequence of electrical pulses. A receiver of this type is not ideal, in practice; that is, the bandwidth of electronic processing is limited. The decision unit provided at the receiver requires a certain amount of time to distinguish between a logic one and a logic zero of the pulses, so that the time base of the receiver (such time base being synchronized with the clock of the received signal), has a certain amount of jitter. High-bit-rate data in the received signal, thus is not read correctly.
An object of the present invention is therefore, to provide a bandpass filter which has optimum spectral efficiency in high-bit-rate optical transmission systems, preferably with regard to optical reception. The bandpass filter should, in particular, be suitable for filtering one or more channels in a WDM or DWDM signal.